Nitrogen oxide (NOx), sulfur oxide (SOx), dust and the like generated by combustion of fossil fuel in a thermal power plant are contained in exhaust gas (flue gas). Such NOx, SOx, dust and the like cause air pollution, and therefore a NOx removal apparatus that removes NOx and a flue gas desulfurization for removing SOx are provided in a thermal power plant and the like having a coal combustion boiler. A wet limestone/gypsum method predominates in such a flue gas desulfurization installed in thermal power plants. In this technique, a spray method in which a limestone slurry is sprayed from a nozzle to bring the limestone slurry into gas-liquid contact with flue gas has high reliability, and has been frequently adopted as a spray method of the limestone slurry into a column.
In a conventional flue gas desulfurization adopting the spray method, flue gas from a boiler or the like is introduced from a gas inlet duct into an absorber, limestone as an absorbent of sulfur oxide is supplied to the desulfurization as a limestone slurry, and the absorbent is sprayed from spray nozzles installed in the absorber.
Flue gas is introduced from an opening provided on a side face or the like of the absorber to flow toward an opening provided in an upper part of a column of the absorber. An absorbent containing limestone slurry is sprayed from each nozzle installed in the absorber toward flue gas flowing in the column. Gas-liquid contact is made between flue gas supplied into the absorber and droplets of the absorbent, and SOx in flue gas is removed on the surfaces of the droplets of the absorbent along with NOx, dust, and acidic gas such as hydrogen chloride and hydrogen fluoride. Mist entrained in flue gas is removed by a mist eliminator, reheated as required, and then discharged from a stack through a gas outlet duct provided at the top of the column as clean flue gas.
The absorbent having absorbed SOx accumulates in a lower part of a main body of the absorber. SOx removed from flue gas in the absorber reacts with limestone in the absorbent to become calcium sulfite as an intermediate product. Air for oxidation is supplied to the absorber by a blower or the like, so that calcium sulfite is oxidized by oxygen in the air supplied into the absorber, thereby generating calcium sulfate (gypsum).
In a wet-type FGD (Flue Gas Desulfurization) adopting a wet-type limestone/gypsum process, the limestone reactivity decreases due to NOx, SOx, dust and the like contained in flue gas flowing into the absorber.
Therefore, for example, in the conventional flue gas desulfurization, absorbing activity of the absorbent is increased by feeding an absorbing agent having higher absorbing activity with respect to NOx, SOx, dust and the like such as magnesium hydroxide (Mg(OH)2), sodium hydroxide (NaOH) or the like into a desulfurization absorber (see, for example, Patent Literature 1).